Sun Angles

Fall leaves, Zion National Park, Utah

Fall leaves, Zion National Park, Utah

The arrangement of the earth and sun are important in seasonal and daily changes in solar radiation. Radiation has a large influence upon ecosystems. Look around as you hike. The north and south sides of hills and mountains have different vegetation. Tree line is higher on south facing slopes (in the Northern Hemisphere). South facing slopes have less snow accumulation and melt off more quickly in the spring. In dry climates the north facing slopes have more lush vegetation. This is all caused by differences in solar radiation input.

Solar radiation on a flat surface in watts/square meter

Figure 1. Solar radiation on a flat surface in watts/square meter.

Figure 1 shows radiation levels on a flat surface based upon time of year (January 1 = 1; December 31=365) and time of day. Days are longer in the summer and noon time radiation peaks at a higher level.

Figure 2. Radiation on a south facing slope in watts/square meter.

Figure 2 is for a south facing slope. In comparison to the flat area, radiation levels change much less throughout the year. In fact noon solar radiation is actually higher in the winter than it is in the summer on steep south facing slopes. Often plants can grow to the north of their normal range on steep south facing slopes.

Figure 3. Radiation on a north facing slope in watts/square meter.

Figure 3 gives interesting results for a north facing slope. During much of the winter the north facing slope receives no direct solar radiation. During the summer the radiation levels are low, but the period of time that the sun is shining on the slope is greater on the north slope than on the south slope. Even a purely vertical north face receives direct sunlight; but only in the summer during the morning and afternoon (but not at noon).

 


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